Dopamine Receptors Dopamine plays a major role in the regulation of cognitive, emotional and behavioral functions abnormalities in its regulation have been implicated in neuropsychiatric and substance use disorders. Dopamine receptors belong to G protein-coupled receptor (GPCR) family, and are implicated in various vital physiological functions. That D3R expression is elevated in response to drugs of abuse, has prompted efforts toward the development of D3R-selective agents for the treatment of drug addiction. Inhibition of D3R may be less prone to causing motor side effects that can result from D2R blockade. Partial agonists of D2R have the advantage of not causing the side effects elicited by antagonists. The receptor-ligand interactions that determine the intrinsic efficacy of such drugs, however, are poorly understood. Aripiprazole has an extended structure comprising a phenylpiperazine primary pharmacophore and a 1,2,3,4-tetrahydroquinolin-2-one secondary pharmacophore. We combined site-directed mutagenesis, analytical pharmacology, ligand fragments, and molecular dynamics simulations to identify the D2R-aripiprazole interactions that contribute to affinity and efficacy. We reveal that an interaction between the secondary pharmacophore of aripiprazole and a secondary binding pocket defined by residues at the extracellular portions of transmembrane segments 1, 2, and 7 determines the intrinsic efficacy of aripiprazole. Our findings reveal a hitherto unappreciated mechanism for fine-tuning the intrinsic efficacy of D2R agonists. The discovery of functionally biased and physiologically beneficial ligands directed toward GPCRs has provided the impetus to design D2R targeted molecules that may be therapeutically advantageous for the treatment of certain neuropsychiatric or basal ganglia related disorders. For a novel series of D2R agonists linking the D2R unbiased agonist sumanirole with privileged secondary molecular fragments, our extensive in vitro functional studies and bias factor analysis led to the identification of a novel class of highly potent Go-protein biased full D2R agonists with more than 10-fold and 1000-fold bias selectivity toward activation of specific G-protein subtypes and -arrestin, respectively. Intracellular electrophysiological recordings from midbrain dopamine neurons demonstrated that Go-protein selective agonists can elicit prolonged ligand-induced GIRK activity via D2Rs, which may be beneficial in the treatment of dyskinesias associated with dopamine system dysfunction. Dopamine and Serotonin Transporters DAT and SERT belong to the Neurotransmitter:Sodium Symporter (NSS) family, and serve to terminate dopamine and serotonin neurotransmission respectively, by recycling released neurotransmitters back into the presynaptic neuron. DAT is the primary target for abused psychostimulants such as cocaine and methamphetamine, whereas 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) binds to SERT. An understanding of the full spectrum of functional states and their transitions in a transporter cycle is required to understand the functions of these proteins and the complexity of the ligand binding modes, in order to identify and eventually exploit the therapeutic opportunities in reducing the efficacy of the abused drugs. Thus, the varied inhibition mechanisms of inhibitors are of particular interest in developing targeted and effective therapeutic interventions for drug abuse and other psychiatric disorders. Previous efforts to develop potential pharmacotherapies for cocaine use disorders have led to the identification of a promising lead molecule, JJC8-091, that demonstrates a novel binding mode at DAT. JJC8-091 and a structural analogue, JJC8-088, were extensively and comparatively assessed to elucidate neurochemical correlates to their divergent behavioral profiles. Computational models predicted that JJC8-088 binds in an outward facing conformation of DAT, similar to cocaine. Conversely, JJC8-091 steers DAT towards a more occluded conformation. These data reveal the underlying molecular mechanism at DAT that may be leveraged to rationally optimize leads for the treatment of cocaine use disorders, with JJC8-091 representing a compelling candidate for development. Although recent hSERT crystal structures represent a milestone for structure-function analyses of mammalian NSS, they are all derived from thermostabilized but transport-deficient constructs. Two of these structures are in complex with paroxetine, the most potent selective serotonin reuptake inhibitor known. By carrying out and analyzing the results of extensive and comparative molecular dynamics simulations while also re-evaluating the transport and binding properties of the thermostabilized constructs, we identified functionally important structural elements that are perturbed by these mutations, revealed unexpected dynamics in the central primary binding site of SERT, and uncovered a conceivable ambiguity in paroxetine's binding orientation. We propose that the favored entropy contribution plays a significant role in paroxetine's extraordinarily high affinity for SERT. Our findings lay the foundation for future mechanistic studies and rational design of high-affinity SERT inhibitors. Sigma 1 Receptor The 1R is a structurally unique transmembrane protein that functions as a molecular chaperone. It is located at the mitochondria-associated endoplasmic reticulum (ER) membrane (MAM), and has been found to translocate to the plasma membrane and other parts of the cell, and modulates the functions of targets relevant to drug abuse such as dopamine receptors and DAT. Dysfunctions of such modulations are connected to many neurological disorders. In particular, 1R has been implicated in cocaine abuse. Cocaine shows biochemical affinity and pharmacological activity at both DAT and 1R. It has been demonstrated that compounds that can antagonize the action of cocaine at both sites may have therapeutic potential of cocaine abuse. 1R agonists can substitute for cocaine in self-administration, and the antagonists can block the self-administration interestingly, a few DAT inhibitors have high affinities for both DAT and 1R. However, the 1R pharmacology and the synergy of the ligands on other targets, in particular DAT, are ill-defined at molecular and cellular levels. In addition, mechanistic underpinnings of structure-function relationships of 1R are poorly understood, and molecular interactions of selective ligands with 1R have not been elucidated. Specifically, outstanding issues surrounding the functional or pharmacological relevance of 1R oligomerization remain, such as the minimal protomeric unit and the differentially altered oligomerization states by different classes of ligands. Western blot (WB) assays have been widely used to investigate protein oligomerizations. However, the unique topology of 1R renders several intertwined challenges in WB. We developed a WB protocol without temperature denaturization to study the ligand binding effects on the oligomerization state of 1R. Using this approach, we observed unexpected ladder-like incremental migration pattern of 1R, demonstrating preserved homomeric interactions in the detergent environment. We compared the migration patterns of intact 1R construct and the C-terminally tagged 1R constructs, and found similar trends in response to drug treatments. In contrast, N-terminally tagged 1R constructs show opposite trends to that of the intact construct, suggesting distorted elicitation of the ligand binding effects on oligomerization. Together, our findings indicate that the N-terminus plays an important role in eliciting the impacts of bound ligands, whereas the C-terminus is amenable for modifications for biochemical studies.